The present invention relates to a method of calibrating O2 sensors used in the exhaust systems of internal combustion engines. More particularly, the invention relates to Zirconia-based O2 sensors, such as those used in diesel exhaust systems.
In general, internal combustion engines need a specific air-to-fuel ratio (or ratio range) to operate correctly. For gasoline engines, the ideal ratio is 14.7 parts of air to one part of fuel. When the ratio is less than 14.7, not all fuel in the air-fuel mixture is burned or combusted. This situation is referred to as a rich mixture or rich condition and has a negative impact on exhaust emissions because the leftover fuel becomes pollution in the form of hydrocarbons (“HCs”) and carbon monoxide (“CO”). When the air-fuel ratio is less than 14.7, excess oxygen is present in the air-fuel mixture. This situation is referred to as a lean mixture or lean condition. When an engine burns lean, it produces nitrogen-oxide pollutants and, in some cases, engine performance decreases, engine damage occurs, or both events occur.
In modern engines, the air-fuel mixture is controlled, in part, through use of an O2 sensor. The O2 sensor communicates with an engine control unit in a feedback loop which typically either controls a fuel quantity or an Exhaust Gas Recirculation (“EGR”) rate. In some engines, the engine control unit uses the O2 sensor's input to adjust the fuel mixture. The O2 usually sensor measures the oxygen level inside the exhaust manifold.
The performance of an O2 sensor degrades when its exhaust gas inlets ports become fouled or blocked. This blockage could occur due to being coated with oil or by being covered with an exhaust by-product such as soot. The performance of an O2 sensor can also degrade due to age. Typically, when a sensor ages it produces an incorrect signal or no signal at all. A properly operating O2 sensor (e.g., one used in a gasoline engine, located upstream of the catalytic converter, and not aged or contaminated or blocked by soot or other combustion by-products) should fluctuate between a rich and lean mixture at least once a second to keep the amount of harmful emissions low. A properly working O2 sensor in a diesel application should provide an output or reading that changes with changes to engine loading (within a reasonable amount of time).
The result of aging in a Zirconia-based O2 sensor is typically manifested in signal drift and incorrect sensor readings. To compensate for signal drift, an overrun adaptation is used. “Overrun” refers to a situation in which a vehicle's exhaust pipe is purged with air from outside the engine. As is known, atmospheric air has an oxygen content of about 21%. If an O2 sensor reading (after signal pressure compensation) in overrun differs from 21%, the engine control unit assumes that the deviation is due to aging of the sensor. The common procedure is to compensate for this signal deviation by determining a correction factor which is then applied to all following readings. In the next overrun cycle, the correction factor can be trimmed or adjusted to account for sensor aging between the current and past overrun cycles.